How a remote controlled car know which button is pressed?

Let us assume a remote controlled car which operates at 2.4GHz frequency band. When the accelerate or left or right turn button is pressed what value changes so that the car detects which button was pressed?

I don't recall them ever using true Pulse Position Modulation although that is what they call it.....

In the early days they used AM radios to send audio tones. These were fed to a device called a reed bank. This was a bit like a relay with a comb on top. The length of each tine was different so it only vibrated if the right audio tone was sent. In this way multiple channels could be decoded out to control different functions.

In the 1960s when I started they used AM radios (typically on the 27MHz band) and the servo or switch position was indicated using pulse length modulation. A 1mS pulse indicated (say) full left, 2mS = full right, 1.5mS = in the middle. These variable length pulses were grouped into frames. A pulse was sent for each channel/servo in turn with a 0.5mS gap between each. Then a longer pulse was sent to indicate the start of a new frame (sync pulse). Frames were sent at about 50 per second. These days people call this Pulse Position Modulation but I think it would be more accurate to call it Pulse Length Modulation. I suppose it is Pulse Position if you look at it from the perspective of the gaps rather than the pulses.

Then came FM radios which still used Pulse Length Modulation.

Then came FM radios which used Pulse Code Modulation. I think the early sets used 8 bits to represent the position of each channel meaning they resolved the control stick position into one of 256 digital values (0-255). That was soon followed by sets using 10 bits/1024 values per channel. PCM sets were typically the first to use microcontrollers to provide additional functions such as mixers (for example for V tail aircraft).

Modern sets are spread spectrum and I think they use QPSK (Quadrature Phase Shift Keying) to send PCM data. I think some also include error correction but I'm a bit out of date.

PPM is still used with modern 2.4GHz transmitters and receivers for powered models. Though the supporting hardware and interface systems have changed hugely over the years, the basic standard for controlling servos has not. There have been speed options added to the PPM interface to improve response and reduce lag, but most servos are backward compatible. They can still take the old standard 50Hz signal even though they can support higher frequencies.

What's changed for the most part is how the transmitter interfaces with the receiver. That is not backward compatible. These days you have to buy a transmitter and receiver as a matched set then do a "bind" procedure to mate them. This is contrary to older systems where any transmitter would work with any receiver. The reason modern transmitters operate this way is to remove any possibility of the receiver responding to signals from another transmitter on the same frequency. Prior to that it was necessary to for operators to coordinate frequency usage with a removable crystal in the transmitter. That was to eliminate interference when more than one model was in operation. People sometimes made mistakes and then a model would go off out of control. That's no longer a concern and it's pretty much idiot proof now.